1. Field of the Invention
The present invention generally relates to a power control device, and more specifically to a power control device for dynamically adjusting the frequency by using a feedback loading signal to determine the level of loading and detect the optimal valley of the power sensing signal so as to change the switching signal which controls the switching transistor based on the optimal valley.
2. The Prior Arts
For many of various electronic devices, the requirements of electric power are quite different, such as direct current (DC) or alternating current (AC) power, different voltage level, current, and electric power. As a result, the power supply manufacturers have been developing many kinds of power conversion device to meet respective needs. For instance, electric motors generally need 12V DC to drive such that the power conversion device has to convert 110V AC city power or lower level electric power provided by the battery into 12V DC power. Many ICs (integrated circuits) are generally designed for 5V, 3V or 1.8V, so city power has to be converted into suitable lower level power. For some application field of high power devices fed with higher level power, such as the inverter supplying higher AC power to turn on the lamps, the power conversion devices have to convert 12V DC power from the battery into 110V or much higher voltage AC power.
In the prior arts, the switching power converter is one of the common used power conversion devices because of the advantages like simple architecture, low cost and broader adjustable range. Especially, the scheme of zero voltage switching and/or zero current switching is used to fulfill the object of reducing the switching loss caused by the switching elements (generally the power transistors) so as to improve the efficiency of power conversion.
Specifically, the switching power converters primarily generate the PWM (pulse-width modulation) signal served as the driving signal to drive the switching transistor, which controls the current to flow through the induction coil wound around the electric transformer, such that the voltage and/or current of the output power is changed. Owing to the inductance effect resulted from the induction coil and the parasitic capacitance effect from peripheral capacitors, electronic elements or loading, the switching transistor may suffer from damping resonance or bouncing after a period of time when turned off. Additionally, to reduce the switching loss, it often needs to switch the switching transistor at some specific valley in the bouncing process. The scheme is also called “valley switching”, which turns on the switching transistor at the specific valley. The operation of valley switching is usually performed at a fixed valley number. That is, the switching transistor is always turned on at the default fixed valley number.
However, one shortcoming of the valley switching with the fixed valley number in the prior arts is that the frequency of the switching signal becomes higher as the loading is lighter, and the frequency is lower as the loading is heavier. Thus, it greatly deteriorates the efficiency of power conversion without dynamically adjusting the frequency of the switching signal according to the actual level of loading.
Therefore, it is greatly needed to provide a power control device for dynamically adjusting frequency, which utilizes a specific control criterion to determine the valley number for the switching signal to change such that the frequency of the switching signal becomes lower when the loading is lighter, the switching loss is greatly reduced, and the efficiency of power conversion is improved, thereby overcoming the above problems in the prior arts.